This month before, Google launched a new pretty large ad size format for Adsense with the 300X600. The reply to the new format looked pretty great, but might be it was too good because the new ad Adsense format, as Google said, is "brand friendly" and is also generally called a half page unit and Google has made a change to its adsense policy to check adsense publishers only use one of these units per page, at most.
This update before, publishers began putting up to three of these ad units on their website pages. Leaving nearly no home for content.
So, after that Google released a statement about changes in Google Adsense policy to just give the permission of the 300x600 ad units on a page at a time. You add three ad units on a site page, but use only one 300x600 unit is granted on the page.
By chance you added multiple, you have only more than half month to remove it. The last of this is January 10, 2013.
Everybody knows that Google doesn’t like website pages which don’t have enough quantity of content at “above the fold” then Google update the algorithm called Google Page Layout Algorithm.